Electronic devices often include an oscillator to provide an oscillating signal for use as a clock source. The oscillating signal can be controlled by a resonator which requires some form of excitation signal to sustain oscillations.
The operation of an oscillator, controlled by the resonator, may be affected by age and certain environmental conditions such as temperature and, of particular interest here, acceleration. When an oscillator is subjected to acceleration, the frequency of the oscillating signal it produces may be altered. The change in frequency is proportional to the magnitude of the acceleration and dependent on direction, giving rise to an acceleration sensitivity vector. A time variable acceleration, for example vibration, can cause frequency modulation of the oscillator's frequency. Reducing the sensitivity of such oscillators to alterations of frequency due to acceleration is therefore desirable in order to produce a stable and pure frequency output from an oscillator.
Known techniques for reducing the sensitivity of oscillators to acceleration may involve packaging and/or resonator design and compensation techniques.
Considering first packaging, this typically involves configuring the packaging so as to achieve a symmetrical crystal resonator design. It has been demonstrated to a first approximation that a theoretical zero exists for the acceleration sensitivity when considering a perfectly symmetric bulk acoustic wave (BAW) resonator and mounting structure. However, achieving a symmetric mounting structure which maintains symmetric stress on the resonator can be a difficult task due to manufacturing imperfections. The configuration of the structure also imposes further problems in terms of stress on the resonator. Spring clips may typically be employed to mount the resonator, so as to reduce stress applied from the surrounding enclosure, but performance may be degraded by resonance of the spring clips themselves, which in turn may lead to amplification of any applied acceleration. Such techniques thus involve relatively large, elaborate designs and complex manufacturing methods.
Turning now to compensation techniques, these mainly fall into three categories: i) mechanical, ii) active and iii) passive. The first category of compensation techniques includes mechanical vibration isolation or dampening techniques. These tend to only affect the upper portion of the vibration frequencies associated with standard conditions, i.e. the upper portion of the ˜0 to 3 kHz frequency range. Compensating in this manner in the lower portion of the ˜0 to 3 kHz frequency range becomes increasingly difficult due to the necessary increase in size of the mechanical isolation system required to filter out the larger amplitudes. Moreover, the vibration isolation system is itself a resonant structure, which can amplify the vibration at and below its resonant frequency and is often only effective along one direction.
The second category of compensation techniques involves passive devices where multiple resonators are located to form a composite oscillator in such a way as to cancel out the effects of acceleration. This is demonstrated for example in U.S. Pat. No. 4,410,822 and also in U.S. Pat. No. 4,575,690 where individual crystal resonators are manufactured and then analysed in order to ascertain their acceleration sensitivity vectors. Once their acceleration sensitivity vectors are known, matching resonators can be selected and mounted in an anti-parallel arrangement in an attempt to try and cancel out the respective acceleration sensitivity vectors of the resonators. Such techniques suffer from the impracticalities of having to measure, select and precisely mount the resonators.
The third category of compensation techniques involves so-called active devices, where one or more accelerometers are used to sense the applied acceleration. The applied acceleration signal can then be used to control a frequency modulation circuit to cancel out the frequency changes induced in a resonator from the applied acceleration. This approach suffers from complications concerned with matching the response of the accelerometer to the resonator over a wide range of vibration frequencies, obtaining accurate accelerometer alignment, and also applying the correction signal to the oscillator frequency in a linear manner.
To overcome the impracticalities encountered with the above compensation techniques, multi-oscillator arrangements have been proposed, for example the arrangement described in U.S. Pat. No. 5,250,871, where four or more resonators are electrically connected in series, with their acceleration sensitivities aligned such that at there are at least two pairs of opposing vectors in each plane. Effectively the “law of large numbers” is used to overcome unit-to-unit variations, so that the more units, the more likely it is that their acceleration sensitivity vectors will cancel each other out. Such techniques are inherently costly, both financially due to the number of units, and in terms of space required for the multi-oscillator arrangements.
FIG. 1 shows side 2 and plan 4 views of a known resonator and package. The oscillator contains a strip resonator 12 mounted to a substrate 16 by a pair of mountings 20. The mountings 20 provide mechanical support and allow for electrical coupling of the resonator. The electrical connections may be in the form of excitation electrodes located above 18a, and below 18b, the resonator to sustain its oscillations. The resonator 12 may for example include a piezoelectric material such as quartz crystal. The resonator 12 is enclosed within a housing 22 for providing mechanical and environmental protection for the resonator. The form, material and method of adhering the housing components to each other and to the substrate is well known in the art, and includes hermetic packaging which may consist of sealing via a Kovar seal ring and seam welded lid. The substrate may for example be made of a ceramic material.
Each resonator has an active resonance region, which is the area defined by electrodes 18a and 18b. It is desirable that this area should be free from any mechanical restriction in order to allow the resonator to function correctly. As such it is desirable to isolate the resonators from as much mechanical stress as possible. To help achieve this, the resonator can be mounted in a cantilever fashion, with the active resonance region remote from the mounting region. These structures are robust, provide good decoupling of mounting strain, and are relatively straightforward to assemble. The cantilevers may be mounted using elements that naturally aid the relaxation of the stress between the package and the resonator, for example using compliant conductive adhesive. A limitation with such resonators is that the magnitude of the acceleration sensitivity vector along the length of the resonator can be relatively high. So, whilst cantilever-mounted resonators may be suited to situations in which compactness and flatness requirements are paramount, and in which relatively high alterations of resonant frequency due to acceleration can be tolerated, a cantilever-mounted resonator is far from ideal in applications requiring low sensitivity to acceleration.
FIG. 1A shows a perspective view of a strip resonator 200 with mountings 206. An active resonance region is defined by electrodes 202 located on either side of the resonator (connections not shown). The orthogonal x, y and z axes of the resonator are shown by coordinate system 204.
FIG. 1B shows a side view of resonator 200 with upper and lower electrodes 202 and mountings 206. When the resonator is subjected to acceleration in the z direction, i.e. along the z axis 208, stress is applied to the resonator through the mountings 206. The stress applied to the resonator varies across the profile of the resonator in the z direction, leading to a so-called non-uniform stress distribution, which is most acute in the region around the mountings 206. The non-uniformity of the applied stress due to acceleration decreases approximately exponentially with distance along the length of the cantilever away from the mountings (in the y direction). This means that the non-uniformity of the applied stress in the z direction decreases approximately exponentially with distance from the mountings to the free end of the resonator, i.e. with distance along the y axis, shown by graph 214. Therefore at a position 210 relatively close to the mountings, the stress applied to the resonator will be relatively non-uniform in the z direction, as indicated by arrows of differing lengths. At a position 212 relatively far from the mountings, the stress applied to the resonator will be relatively uniform in the z direction, as indicated by arrows having the same length. This effect is particularly pronounced in relation to small dimensions of resonator, which in this case is the z dimension (this being very small compared to the y and x dimensions of the resonator).
A similar effect occurs when the resonator is subjected to acceleration in the x direction. FIG. 1C shows a plan view of resonator 200, electrodes 202 and mountings 206. When the resonator is subjected to acceleration in the x direction, i.e. along the x axis 220, stress is applied to the resonator through the mountings 206. The non-uniformity of the applied stress in the x direction decreases approximately exponentially with distance from the mountings, i.e. with distance along the y axis, shown by graph 226. Therefore at a position 222 relatively close to the mountings, the stress applied to the resonator will be relatively non-uniform in the x direction, as shown by arrows of differing lengths. At a position 224 relatively far from the mountings, the stress applied to the resonator will be relatively uniform in the x direction, as indicated by arrows having the same length.
The stress applied uniformly along the edge of the resonator active area in the x and z directions leads to equal and balanced areas of tension and compression. These tend to cancel out any induced resonant frequency change and so lead to a low acceleration sensitivity.
The situation is different when the resonator is subjected to acceleration in the y direction as the y direction is aligned with the direction between the mountings and the resonator active area instead of being orthogonal thereto, as in the case for the x and z directions. FIG. 1D shows a plan view of resonator 200 with electrodes 202 and mountings 206. When accelerated in the y direction by a force applied from the end of the resonator where the mountings 206 are located, the non-uniformity of the applied stress in the y direction does not decrease exponentially with distance from the mountings along the y direction. In this case, the whole of the resonator is in compression, the compressive strain ranging from a maximum at the mountings to a minimum at the free end of the resonator. Similarly, when accelerated in the y direction by a force applied from the free end of the resonator, the whole of the resonator is in tension, the tension strain ranging from a maximum at the mountings to a minimum at the free end of the resonator. As a result there is a non-uniform and unbalanced stress distribution in the y direction across the active resonance region, as indicated by arrows 230 of differing lengths, and either the whole of the active region is in compression, or the whole of the active region is in tension.
In either of these cases, the applied stress changes from being non-uniform in the x and z directions at a position 240 relatively close to the mountings to being relatively uniform in the x and z directions at a position 242 relatively far from the mountings, where the active resonator region is located. In the y direction though, the applied stress is not uniform across the active resonance region of the resonator. In fact, the applied stress changes approximately linearly with distance along the resonator due to the linear change in resonator mass remaining to be accelerated, as indicated by arrows 230 of differing lengths.
This imbalance between areas of tension and compression over the active resonator region leads to an induced resonant frequency shift and hence a relatively high acceleration sensitivity in this y direction.
It would thus be desirable to implement a low-cost solution that reduces the acceleration sensitivity of an oscillator without the need for measurement, selection and precise specific individual alignment of resonators.